If the 'other family members' are the close family who are in fact within the 'Tolkien Estate', then it makes perfect sense.

And you're right, just because they happen to have Tolkien's name doesn't mean their private lives should be in the public domain. I suppose the publishers of the book about Hilary knew that many people would be interested primarily because of the link to JRRT. If they had been willing to take out the stuff that relates to him and his family too directly, it appears there would have been no problem with publication. I suppose the question is, was that stuff essential to tell Hilary's story? Or was it there mostly to draw in people who are interested in his brother? Since we'll never see it, I guess we'll never know... They went in, and Sam shut the door. But even as he did so, he heard suddenly, deep and unstilled, the sigh and murmur of the Sea upon the shores of Middle-earth. From the unpublished Epilogue to the Lord of the Rings